1. Field of the Invention
This invention relates generally to operational amplifier (op-amp) slew rate boosting schemes, and more particularly to a technique to boost the slew rate of an op-amp which has a closed-loop gain that is equal to or very close to unity.
2. Description of the Prior Art
Folded cascode op-amps having a class `AB` buffer output stage and a closed-loop gain that is equal to or very close to unity are well known in the prior art. One such op-amp 100 is illustrated in FIG. 1. The folded cascode op-amp 100 can be seen to have a class `AB` buffer 102 at its output. The equations which govern the operation of the op-amp 100 are also very well known and have been documented by Paul R. Gray and Robert G. Mayer in Analysis and Design of Analog Integrated Circuits, pp. 646-onward, John Wiley & Sons, Inc., 3.sup.rd Edition. It has been shown that if the op-amp 100 is to be made stable, one must control its unity gain crossing frequency. The conventional technique for controlling this unity gain crossing frequency is to couple a compensation capacitor 104 between a high impedance node 106 of the output buffer 102 and an AC ground 108. The frequency where the open loop gain falls to unity for op-amp 100 is then defined as: ##EQU1##
where G.sub.m is the transconductance of the input stage 110 and is determined by the tail current I.sub.tail shared by the input stage 110 transistors. Specifically, G.sub.m is defined as: ##EQU2##
The slew rate (maximum rate of change in output voltage for large input signals) for op-amp 100 is defined as: ##EQU3##
where it is well known the compensation capacitor C.sub.comp 104 must be set to a sufficiently large value in order to make the op-amp 100 stable. If more than two poles exist before the unity gain frequency f.sub.unity is reached, the op-amp 100 will be unstable. Specifically, the compensation capacitor C.sub.comp 104 operates to push the first dominant pole down to a low enough frequency such that the op-amp 100 gain falls below unity before the second pole is reached. Setting the value of compensation capacitor C.sub.comp 104 to achieve the above desired stability characteristics therefore also establishes the maximum slew rate for the op-amp 100 as set forth in equation (3) above. It is, of course, desirable to have a very high slew rate. In view of the foregoing, it can be appreciated that achieving a very high slew rate requires reducing the value of the compensation capacitor C.sub.comp 104. Reducing the value of the compensation capacitor C.sub.comp 104 however, makes the op-amp 100 less stable since the first dominant pole will then be moved to a higher frequency as discussed herein before. The desired value of the compensation capacitor C.sub.comp 104 and the desired high slew rate are therefore in direct conflict with one another.
One conventional technique used to address the direct conflict between the desired value of the compensation capacitor C.sub.comp 104 and the desired high slew rate includes reducing the value of transconductance G.sub.m associated with the input stage 110 be inserting resistors into the emitter paths of the input stage 110 transistors. A lower value for the compensation capacitor C.sub.comp 104 can then be used to achieve amplifier stability as seen by equation (1) above, which also then increases the slew rate as seen by equation (3) above. This technique however, is problematic in that the emitter resistors added to the emitter paths of the input stage 110 transistors introduce additional noise that cannot be tolerated in specific applications such as illustrated in FIG. 2 that simply illustrates unity gain op-amp 100 driving a load 102 that is connected to ground in response to an input signal such as might be used to accommodate ADSL systems.
In view of the foregoing, a need exists for a technique to boost the slew rate of amplifiers having a closed-loop gain at or very near to unity without introducing additional noise into the system.